Unix Files Types

8/12/2019 Unix Files Types

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UNIX FILES

Department of MCA PESIT, UnixSystem Programming


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Files are the building blocks of any OS.

When you execute a command in UNIX, the kernel fetchesthe executable file from the file system and also any filesrequired for reading and writing.

Files in UNIX and POSIX cover a wide range of file types.

They also provide a common set of interfaces to files.

This chapter covers

File Types

File attributes and their uses.

Kernel and process specific data structures used for filemanipulation.



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File Types

Regular Files

Directory File

Character Device File

Block Device File

FIFO file

link file



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Regular File

It can be a text or binary file

No distinction is made between these files.

They can be created, browsed and modified byvarious means such as text editors and compilers.

They can be removed by commands like rminUNIX.



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Directory File

It is like a file folder

It provides a means for users to organize their files intosome hierarchical structure based on file relationship or uses.

Eg: /bindirectory contains executable programs such as cat,rm, sortetc.

create a directory file mkdircommand.

A directory is empty if it contains nothing other than .and

.. files.Remove a directory rmdircommand

Display contents lscommand



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Device Files

Block device files represent a physical device that transmitsdata a block at a time.

Eg: hard disk drives, floppy deivce drives.

Character device files represent a physical device thattransmits data in a character-based manner.

Eg: line printers, modems and consoles.

An application can perform read and write on device files like

on regular files and the OS will automatically invoke appropriatedevice driver to perform actual data transfer.

create a device file mknodcommand.



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mknod /dev/cdsk c 115 5

115 major device number: An index to a kernel table thatcontains the address of device driver functions known to thesystem.

5minor device number: An integer value to be passed as an

argument to a device driver function when it is called. It tellswhich actual device it is talking to.

cCharacter device file

bBlock device file Department of MCA PESIT, UnixSystem Programming


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Device Files Examples/dev/dsp

Digital Signal Processor. It forms the interface between software which produces soundand your soundcard. It is a character device on major node 14 and minor 3.

/dev/fd0

The first floppy drive. It is a character device on major node 2 and minor 0.

/dev/lp0

The first parallel printer device. Subsequent printers are numbered lp1, lp2 etc. They are

character devices on major mode 6 and minor nodes starting at 0 and numbered

sequentially.

/dev/psaux

The PS/2 mouse port. This is a character device on major node 10, minor node 1.

/dev/pcd0

Parallel port CD ROM drives. All are block devices on major node 46. /dev/pcd0 is on

minor node 0 with subsequent drives being on minor nodes 1, 2, 3 etc.Department of MCA PESIT, UnixSystem Programming


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FIFO File

It is a special pipe device file which provides temporarybuffer for two processes to communication by writing/readingdata from the buffer.

Size of the buffer is fixed to PIPE_BUF.create a FIFO file mkfifocommand

mkfifo /usr/prog/fifo_pipe

mknod /usr/prog/fifo_pipe p

Remove a fifo file rmcommand.



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Symbolic link file

Supported by BSD UNIX and UNIX System V.4. No support byPOSIX.1

It contains the path name which references another file in either

a local or remote file system. Create a symbolic link file lncommand

ln s /usr/jose/original /usr/mary/slink

cat n /usr/mary/slink

ls l /usr/mary/slink

srrr-- 1 terry 20 Aug 20, 1994 slink->usr/jose/original



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UNIX and POSIX File Systems

root directory /

Absolute path name starts from /

Relative path name may start with a . or ..

A file name may not exceed NAME_MAX characters and pathname PATH_MAX characters.

legal file name characters A to Z a to z 0 to 9.

The path name of a file is called the hard link. Can have oneor more hard links.

ln /usr/foo/path1 / usr/prog/new/n1



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UNIX and POSIX File Systems

/etc Stores system admin files and programs

/etc/passwd Stores all user information

/etc/shadow Stores user passwords (For UNIX System V only)

/etc/group Stores all group info/bin Stores all system programs like cat, rm, cp..etc

/dev Stores all character and block device files.

/usr/include Stores standard header files/

/usr/lib Stores standard libraries

/tmp Stores temporary file.



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UNIX and POSIX FileAttributes

File type

Access permission

Hard link count

UIDGID

File size

Last access time

Last modify time

Last change time

Inode number

File system ID Department of MCA PESIT, UnixSystem Programming


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UNIX and POSIX File Attributes

UNIXcommand

SystemCall

Attributes Changed

chmod chmod Changes access permission, last changetime

chown chown Changes UID, last change time

chgrp chown Changes GID, last change time

touch utime Changes last access time, modification time

ln link Increases hard link count

rm unlink Decreases hard link count, remove the fileif count is zero

vi, emac Changes file size last access time, last

modification time.Department of MCA PESIT, UnixSystem Programming


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Inodes in UNIX

In UNIX System V, a file system has an inode table which

keeps track of all files.

Each inode table is an inode record contains all attributes of a

file including inode no and physical disk address.

Each inode no is unique to a file system only.

An OS does not keep the name of a file in its inode record. The

mapping from file names to inode nos is done through directory

files.



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Inodes in UNIX

To access a file, for example /usr/joe

The UNIX kernel knows the /directory inode noit is kepr in process-

U area.

It will scan the / directory file to find the inode no of usr file.

Once it gets the inode no, it checks if the calling process has permission

to search usr directory.

It then looks for the joe file.

Whenever a new file is created in directory, entry is made in inode table anddirectory file.

Inode tables are kept in their file systems on disk, but the UNIX kernel

maintains an in-memory inode table to contain a copy of the recently accessed

inode records.



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Application Program Interface toFiles

Files are identified by path names

Files must be created before they can be used.

File type UNIX Command UNIX and

POSIX.1 Systemcall

Regular Files Vi, emacs..etc Open, creat

Directory Files mkdir mkdir, mknod

FIFO Files mkfifo mkfifo, mknod

Device Files mknod Mknod

Symbolic Files ln s Symlink



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Files must be opened before they can be accessed by applicationprograms

A Process may open at most OPEN_MAXfiles of any types at anyone time.

readand writesystem callsFile attributes can be queried by the stator fstatsystem call.

File attributes can be changed by the chmod, chown, utimeandlinksystem calls.

File hard links can be removed by the unlinksystem call.

APIs for Files



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Defined in

Struct stat

{

dev_t st_dev /* file system ID */ino_t st_ino /* File inode number */

mode_t st_mode /* Contains file type and access flags */

nlink_t st_nlink /* Hard link count */

uid_t st_uid /* File user Id */gid_t st_gid /* File group Id */

dev_t st_rdev /* Contains major and minor device numbers */

off_t st_size` /* File size in number of bytes */

time_t st_atime /* Last access time */time_t st_mtime ` /* Last modification time */

time_t st_ctime /* Last status change time */

}

stat, fstat or lstat System CallsDepartment of MCA PESIT, UnixSystem Programming


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UNIX Kernel Support for FilesFile tableAll open files

Inode Tablecopy of file inodes most recently accessedFile descriptor TableAll files opened by the Process, OPEN_MAX

OPEN function

1. Search the File descriptor Table for first unused entry.index to theentry is returned to the process.

2. Scan the File table to find an unused entry. If found,

a) File descriptor Table entry will point to file table entry.

b) File able entry will point to inode table entry.

c) File table entry will contain the current file pointer of the open file.d) File table entry will contain an open mode. (read-only, write-only,

read and write)

e) The reference count in the file table entry is set to 1.

f) The reference count of the in-memory inode of the file is increasedby 1. Department of MCA PESIT, UnixSystem Programming

D


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Data StructureFile descriptor Table

File Table

InodeTable

Process Space

r

rc = 1rw

rc = 1

w

rc = 1

rc = 1

rc = 2

xyz

abc

Kernel Space



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Read/write function The file descriptor is the first argument to read/write system

call. Kernel uses the file descriptor to index to the file descriptor

table to find the file table entry. It checks the file table entry to see if the appropriate mode and

permissions are there. Use the file table entry to access the files inode record. Use the file pointer in the file table entry to determine where

read/write should occur.

Checks the file type in inode record and invokes the appropriatedriver function.

lseek system call Invoked to change the file pointer to a different offset for next

read/write operation.

The kernel gets access to the file inode record and checks thatthe file is not a character device file, a FIFO file or a symboliclink file.

If file type is compatible, change the file pointer to the value inlseek.



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Close function

1. The kernel sets the FD entry to be unused2. Decrement the reference count in file table by 1. If still non-

zero, go to 6.

3. The file table entry is marked as unused.

4. Decrement the reference count in file inode table by 1, if stillnon-zero, go to 6.

5. If hard-link count of inode is non zero, return to caller.Otherwise it marks the inode table entry as unused anddeallocates all the physical disk storage of the file, as all the

file path names have been removed by some process.6. It returns to the process with a 0 (success) status.


P d F l


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C Stream Pointers and FileDescriptors

A file descriptor allocated by anopensystem call

A File descriptor is moreefficient for applications that dofrequent randomaccess of filedata. (No I/O buffering)

File descriptors are used only inUNIX and POSIX.1 complientsystems

C Stream pointers (FILE *) areallocated via the fopenCfunction call.

A stream pointer is moreefficient to use for applicationsdoing extensive sequentialreadfrom or write to files. (I/Obuffering)

Stream pointers are supportedon all Operating systems,(VMS, CMS, DOS, UNIX)



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Each process has a fixed-size stream table with OPEN_MAX entries.

FILE : buffer, file I/O error status, EOF flagetc

fopenreturns a reference to this.

Extract the file descriptor associated with a stream pointer.

int fileno( FILE* stream_pointer);

To convert a file descriptor to a stream pointer

FILE* fdopen(int file_descriptor, char * open_mode);

C LIBRARY function UNIX system call used

fopen open

fread, fgetc, fscanf, fgets read

fwrite, fputc, fprintf, fputs write

fseek, ftell, frewind lseek

fclose close



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Directory Files

Directory Function Purpose

Opendir Opens a directory file

Readdir Reads the next recordfrom the file

Closedir Closes a directory file

Rewinddir Sets file pointer tobeginning of file

for UNIX System V and POSIX.1 systems

for BSD UNIX



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Hard Link Vs Symbolic LinkA Hard link is a UNIX path name for a file

ln /usr/mary/abc /usr/mary/xyz

Symbolic links are created with the s option

ln

s /usr/mary/abc /usr/mary/xyz

Limitations of hardlinks:

Cannot create hardlinks for directories unless they have

superuser privileges Users cannot create hardlinks on a file system that

references files on a different system.



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Hard Link Vs Symbolic Link

Hard Link Symbolic Link

Does not create a new inode Create a new inode

Cannot link directories, unless thisis done by the root

Can link directories

Cannot link files across filesystems Can link files across file systems

Increase the hard link count of thelinked inode

Does not change the hard linkcount of the linked inode



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ln Vs cpln creates a new directory entry to a referenced file whereas cp creates aduplicated copy of the file to another file with a different name.

ln /usr/mary/abc /usr/mary/xyz

Inodenumber

Filename

115

89

201 abc

346 a.out

Inodenumber

Filename

515

989

201 Xyz

146 Fun.c

Inodenumber

Filename

115

89

201 abc

346 t

Inodenumber

Filename

515

989

345 Xyz

146 F

(lns)/ cp /usr/mary/abc /usr/mary/xyz


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Unix Files Types